Figures 10 and 11 show the net and
gross irrigation water requirements for potential cropping pattern and potential
irrigation efficiency with effective rainfall. Table 8 summarizes the figures for each of
the 84 zones. NIWR and GIWR for the potential scenario with effective rainfall were
further combined with the 136 basic units of this study to obtain individual NIWR and GIWR
for each of these units through GIS.

The results have been compared with
figures available from country studies (national water master plans, projects, etc.). The
comparison shows that the methodology yields relatively accurate regional estimates of IWR
that are suitable for the present study. Discrepancies with country studies find their
origin mostly in the assumptions made on cropping pattern, cropping intensity and
irrigation efficiency, and are discussed in details in Chapter 6.

The influence of cropping pattern
zones on the quality of the output is of prime importance. Important differences in
irrigation water requirements in adjacent zones is one of the consequences of this
approach. For instance, in Burkina Faso, areas located north of the 1000 mm annual
rainfall line have a gross potential water requirement of 500 mm per year, while areas
located just south of this line need more than 2800 mm per year. This artificial break is
due to the choice of the irrigated cropping pattern zones, where it was decided that no
rice was cultivated under 1000 mm of rainfall per year. Within the cropping pattern zones,
the boundaries of irrigation water requirement zones follow rainfall trends.

The estimates used for cropping
intensity and irrigation efficiencies, to obtain the gross irrigation water
requirements from the net figures, also have a direct influence on the results presented
on the final maps.

The differences in irrigation water
requirements between adjacent zones is directly related to the density of the climate
stations' network. In low-density areas, such as the Sahara and southern Africa,
differences in IWR between adjacent zones are high (up to 600 mm/year gross requirements)
as the low station density does not allow the delineation of HIWR zones with smaller
differences. A high density of the station network in the rest of the continent, in
combination with rainfall raster maps, has resulted in differences of a maximum of 200
mm/year gross requirements between adjacent zones.